There are lost of tools available to scan a network and list the IPs of the hosts that are live.
Here is one that I find quite cool using nmap with the following command(for example for the LAN network 192.168.100.0/24):
Reference: http://security.stackexchange.com/questions/36198/how-to-find-live-hosts-on-my-network
nmap 192.168.100.0/24 -n -sP | grep report | awk '{print $5}'
the result could be for example:
192.168.1.1
192.168.1.11
192.168.1.12
192.168.1.13
192.168.1.14
192.168.1.15
192.168.1.118
192.168.1.122
192.168.1.123
192.168.1.126
192.168.1.129
192.168.1.133
192.168.1.134
192.168.1.156
192.168.1.159
192.168.1.168
192.168.1.170